In accordance with a large-capacity optical disc, shortening of a wavelength of a light source of an optical head used in recording and/or reproducing on the optical disc and increasing of the NA (Numerical Aperture) of an objective lens progress. However, in accordance with increasing of the NA, an influence of a spherical aberration caused by a change in thickness of a light-transmitting layer becomes conspicuous.
For example, when a wavelength of 650 nm and an objective lens having an NA of 0.60 are used in a DVD, a spherical aberration of about 10 mλ is generated with respect to a change of 10 μm in thickness of the light-transmitting layer. On the other hand, when a wavelength of about 400 nm and an objective lens having an NA of about 0.85 are used in a next-generation optical disc, a spherical aberration of about 100 mλ is generated with respect to the change of 10 μm in thickness of the light-transmitting layer. For this reason, a means which corrects the spherical aberration is necessary for such optical head for the optical disc.
A Japanese Unexamined Patent Application Publication No. 11-2509906 describes a system of an optical disc device in which a collimating lens is mounted on an actuator for the collimating lens and the collimating lens arranged between a light source and an objective lens is moved so as to cancel a spherical aberration caused by a thickness error of a light-transmitting layer. The optical disc device will be described below with reference to FIG. 18.
FIG. 18 shows a configuration of a conventional optical head. In FIG. 18, reference numeral 101 denotes a light source; 102 denotes a polarizing beam splitter; 103 denotes a ¼ wave plate; 104 denotes a collimating lens; 106 denotes an objective lens; 107 denotes a multi-lens; 108 denotes a light-receiving element; 109 denotes a biaxial actuator which drives the objective lens 106; and 110 denotes a collimating lens actuator which drives the collimating lens 104. These components constitute an optical head 120.
A laser beam emitted from the optical source 101 passes through the polarizing beam splitter 102 and enters into the collimating lens 104. The incident laser beam into the collimating lens 104 is collimated by the collimating lens 104 when a thickness of a light-transmitting layer 131 of an optical disc 130 is a rated value. The collimating lens 104 is mounted on the collimating lens actuator 110, and can be reciprocally moved along the optical axis of the laser beam by this collimating lens actuator.
The laser beam passing through the collimating lens 104 is transformed to a circular polarization state while the laser beam passes through the ¼ wave plate 103 and then enters into the objective lens 106. The laser beam focused by the objective lens 106 and reached on an information recording surface of the optical disc 130 is reflected by the information recording surface and becomes a return beam. The return beam passes along the former optical path and through the objective lens 106, and then enters into the ¼ wave plate 103. The return beam becomes linearly polarized beam rotated at 90° with respect to a polarizing direction of an outward path while passing through the ¼ wave plate 103. Afterward, the return beam is converged by the collimating lens 104 and reflected by the polarizing beam splitter 102. The return beam reflected by the polarizing beam splitter 102 is reached on the light-receiving element 108 through the multi-lens 107 and detected.
When a beam is focused on the information recording surface of the optical disc 130 with the optical head 120 to perform recording/reproducing, main aberrations generated by a thickness error of the light-transmitting layer 131 of the optical disc 130 are caused by defocusing and a spherical aberration. The defocusing can be corrected by a focus servo operation. More specifically, on the basis of the focus serve operation by the light-receiving element 108, the defocusing is corrected by moving the objective lens 106 in the optical axis direction by the biaxial actuator 109, and the beam is focused on the information recording surface.
On the other hand, with respect to the spherical aberration, the laser beam entered into the objective lens 106 is transformed into a diverged beam or converged beam, so that a spherical aberration having polarity opposing that of the spherical aberration generated depending on the thickness of the light-transmitting layer 131 is generated in order to perform correction. More specifically, the collimating lens 104 is moved forward and backward along the optical axis direction by the collimating lens actuator 110, thus the laser beam reached on the objective lens 106 is changed into the diverged beam or converged beam to cause the objective lens 106 to generate the opposite spherical aberration. Therefore, the spherical aberration caused by the thickness error of the light-transmitting layer 131 is canceled. In this manner, in the optical head 120, when the beam is focused on the information recording surface through the objective lens 106, the spherical aberration is canceled out.
As described above, the method of moving the collimating lens along the optical axis direction to change the laser beam being entered on the objective lens into the diverged beam or the converged beam is used in an optical head for a high-density optical such as a BD (Blu-ray Disc). In a BD having two light-transmitting layers 131, a distance between the light-transmitting layers is 30 μm in consideration fluctuation in the thicknesses of the light-transmitting layers at a maximum.